The invention relates to an optical detection device comprising at least one input aperture, an imaging device, at least one diffraction grating associated with the at least one input aperture and a linear detection array. The invention is also directed to a device for validating bank notes and to a method for the spectral analysis of an area of detection of an object.
Bank note validation (verification or authentication) is usually based, at least to some extent, on an optical detection method (see e.g. U.S. Pat. No. 5,607,040, EP 0 935 223). It is known to perform the verification by comparing a detected pattern with reference patterns (see e.g. EP-A-0947964).
Optical spectroscopy is well known in industrial laboratories and on production lines for process monitoring. In such applications spectrometers measure the optical spectrum in a given area of the object to be analyzed. Generally speaking, there are two ways to do that:
Scanning the spectrum in narrow bands by a single detector or measuring several spectral components simultaneously by a linear array detector.
Imaging spectrographs are already available on the market. An example of such a device is described in OLE, June 1994 pp. 33. This known device is a combination of a spectral analyzer and a CCD camera. Light enters through a slit at the front end of the spectral analyzer. It is then split up into its spectral components by a transmission grating and focussed on a two-dimensional CCD-detector (which is part of a conventional CCD camera). The transmission grating is sandwiched between two dispersing prisms (a construction that is known as PGP: “Prism-Grating-Prism”).
The spectrometers known so far are well suited for production monitoring in industrial processes where plenty of space is available. However, they cannot be readily reduced in size and hence the known construction is not suited for integrated technical solutions.